The present invention relates to an electrophotographic carrier coating agent, in particular a coating agent for use in an electrophotographic carrier which exhibits excellent copying durability and an electrophotographic carrier, the nuclear surface of which is coated therewith.
It is known that the developer for a two-component dry type copier comprises two components, a minute toner and carrier particles which are larger than the toner, where mixing and agitating the two generates friction, which electrostatically charges the toner and carrier to different polarities. The resultant charged toner, when electrostatically adhered to an electrostatic latent image generated on a photoreceptor, generates a visible image; the image is then transferred to and fixed on a transfer sheet to complete the copying operation.
The carrier is normally an oxidized or nonoxidized iron powder. If it is used unmodified with the toner, this results in insufficient triboelectric charge characteristics for the toner, in addition, it results in the toner being bound to the carrier surface during use so as to generate a toner-covered film (spent condition), thereby changing the charge characteristics of the carrier over a period of time and eventually shortening the lifetime of the developer.
An additional shortcoming has been that the difference in carrier charge characteristics between dry and wet environments is large. Therefore, it is customary for the prevention of these drawbacks to coat the carrier surface with resins, such as fluororesins, acrylic resins, styrene-acrylic copolymers, silicone resins, polyester resins, and the like.
However, coating with a hydroxyl-containing polyester resin or an acrylic resin, or the like, has been deficient in that the method requires crosslinking with an isocyanate or melamine, or the like (Japanese Patent Kokai Publications Sho. 59-53875 and Sho. 60-59369), so that unless crosslinking was complete, the unreacted hydroxyl or isocyanate groups tended to remain, adversely affecting the charge characteristics.
Coatings with acrylic resins, styrene-acrylic copolymers, or the like, have been deficient also in that while initial charge characteristics were excellent, the brittle coated films decreased the durability.
On the other hand, fluororesins, silicone resins, etc., have drawn attention because the low surface energies of these resins can reduce the spent condition (Japanese Patent Kokai Publications Sho. 54-21730 and Sho. 58-40557; Japanese Patent Kokoku Publications Sho. 59-26945 and Sho. 59-131944).
Silicone resins provide a number of working advantages in that monomer selection can provide a variety of molecular structures, which gives the advantage of being able to generate carriers with a variety of static charge levels; solubility in a variety of solvents permits a uniform coating over the carrier surface; and curing is possible even at relatively moderate temperatures, etc.
However, silicone resins, in general, are deficient in mechanical strength, though not as bad as with acrylic resins, so that they tend to generate wear, peeling, cracks, and the like, after use over a long period of time, thereby sloughing-off the surface of the carrier and thus, losing their excellent properties and adversely affecting the copying capabilities, so that they have never been completely satisfactory in terms of a long-term use service life.
A long-term service life coating agent has been disclosed in the past (Japanese Patent Kokai Publications Hei. 2-73372 and Hei. 2-103563), but as yet there have been none with silicone-modified acrylic resins.